Field of the Invention
The present invention relates to apparatus and methods for single pass fiber amplification of dissipative soliton-like seed pulses. In particular, the present invention relates to such amplification of seed pulses of >1 picosecond duration that are subsequently compressed to 50-200 femtosecond output pulses.
Discussion of Related Art
Standard fiber amplifiers use the scheme of CPA (Chirped Pulse Amplification) where an oscillator pulse is temporally stretched to tens of ps to tens of ns, amplified, and then recompressed. The main goal of CPA is to avoid nonlinear effects, which can reduce the quality of the final compressed pulse (affecting either the shortness of the pulse or the amount of residual energy, “pedestal”, not in the main short pulse, or both). Traditional CPA using optical fiber based amplifiers is limited in output pulse duration due to a combination of factors including gain narrowing and high order dispersion effects (all dispersion orders greater than second order dispersion). FIG. 1A (Prior Art) illustrates schematically the stretching, amplification, and subsequent compression of a short pulse. The stretchers and compressors in this case can be comprised either of bulk components or optical fibers.
More recently, approaches have been shown to utilize a combination of pulse stretching and nonlinear amplifier propagation to partially compensate for residual high order dispersion terms. See, for example U.S. Pat. Nos. 7,414,780, 8,228,597, 8,503,069, and 9,042,004. This approach, also known as “cubicon amplification” is an amplification process that uses nonlinear effects (self-phase modulation, or SPM) on specially-shaped spectra (triangular spectrum also described as a “sharkfin”) to compensate for third-order dispersion (TOD) in a mismatched stretcher and compressor. The main advantage of this approach is the ability to use an all-fiber stretcher, which simplifies the amplifier design. This type of amplification supports high-fidelity compression of the output laser pulse even in the presence of high cumulative nonlinear phase shifts in the amplifier corresponding to >1 radian nonlinear phase, improving the obtainable output energy over traditional fiber CPA, which is not robust to nonlinear phase shifts. It can also reduce the pulse duration to <350 fs, shorter than with traditional fiber CPA. FIG. 1B (Prior Art) illustrates cubicon amplification. It is similar in form to CPA in that the pulse is stretched in time, however, in this case the pulse is left short enough to accumulate a desired amount of nonlinear phase. This nonlinear phase at least partially compensates for the (non-compensated) third-order dispersion from the stretcher and compressor. This gives rise to the characteristic “sharkfin” spectral shape which has been indicated schematically.
Another approach to overcoming the limitations of traditional CPA fiber laser systems is parabolic amplification, also known as self-similar or similariton amplification. Parabolic amplification is a process where a pulse is stretched and spectrally modified to propagate through the amplifying medium while maintaining the temporal shape, but not the pulse duration/spectral width. Generally, the temporal pulse is very well characterized by a parabola, even near the wings of the pulse, while the spectral shape starts narrow and broadens as the pulse amplifies. This is also called self-similar propagation because for many levels of output pulse energy, the output pulse retains a similar shape (parabolic) and the time duration of the output is proportional to the cube root of the pulse energy. Parabolic amplifiers can sustain high levels of nonlinear phase shifts, while maintaining a nearly quadratic phase profile, which makes pulse compression to near the transform limit easier. FIG. 1C (Prior Art) illustrates the process of parabolic pulse (“similariton”) amplification. In this process, a short optical pulse is stretched, as in CPA and cubicon, but only to such an extent as to allow the formation of self-similar pulses emerging from the amplifier showing their characteristic parabolic temporal shape.
FIG. 1D is a schematic diagram illustrating an All-Normal-Dispersion (ANDi) femtosecond fiber laser. This laser is described in U.S. Pat. No. 8,416,817 (incorporated herein by reference). The ANDi laser produces a “bat ears” shaped output pulse, which is one option for the seed pulse of the present invention.
A need remains in the art for apparatus and methods for single pass amplification of dissipative soliton-like seed pulses with initial and final pulse duration of significantly larger than 1 ps in pulse duration, without requiring a (large footprint) pulse stretcher before amplification, and with output capable of compression using standard (linear) techniques to produce output pulses of 50-200 fs, or in some case 300 fs.